ECG Axis Interpretation

Axis

Definition

  • the mean direction of electrical forces in the frontal plane ( limb leads) as measured from the zero reference point (lead 1)
  • Normal values
    • P wave: 0 to 75 degrees
    • QRS complex: -30 to 90 degress
    • T wave: QRS-T angle <45 degrees frontal or <60 degrees precordial

Measurement

Quick look tests

  • The simplest method of identifying gross deviations in axis is to look at the QRS complexes in leads I and aVF. Lead I is a left-sided lead, and as aVF is perpendicular to lead I, it can be considered a right-sided lead.
    • Both leads I and aVF have mainly positive QRS complexes = normal axis.
    • Lead I is positive and aVF is negative = left axis deviation (LAD).
    • Lead I is negative and aVF is positive = right axis deviation (RAD).
    • Both leads negative = extreme RAD or “North-West” axis
  • Quick look test — Limb leads

 

Measuring ECG axis (simple version 1)

Measuring ECG axis (simple version 1)

  • Quick look test — leads I and aVF

Measuring ECG axis (simple version 2)

Measuring ECG axis (simple version 2)

Isoelectric lead method

  • identify the most isoelectric lead (contains QRS complexes with equal positive and negative deflections)
  • the hearts electrical forces are moving in a direction perpenicular to the isoelectric lead.
  • the axis approximates the direction of the lead perpendicular to the isoelectirc lead if the the QRS deflection is positive in the perpendicular lead.
  • e.g. if lead I is isoelectric (0 degrees) look at aVF (90 degrees). If the QRS complexes are positive in aVF then the axis is approximately 90 degrees.

Interpretation of QRS Axis

  • Normal
    • 0 to 90 degrees
  • Right Axis Deviation (RAD)
    • > 90 degrees
      • moderate RAD: 90 to 120 degrees
      • marked RAD: 120 to 180 degrees
    • Differential diagnosis
      • Right Ventricular Hypertrophy (RVH) — most common
      • Left Posterior Fascicular Block (LPFB) — diagnosis of exclusion
      • Lateral and apical MI
      • Acute Right Heart Strain, e.g. acute lung disease such as pulmonary embolus
      • Chronic lung disease, e.g. COPD
      • Dextrocardia
      • Ventricular pre-excitation (WPW) — LV free wall accessory pathway
      • Ventricular ectopy
      • Hyperkalemia
      • Sodium-channel blockade, e.g. tricyclic toxicity
      • Secundum ASD — rSR’ pattern
      • Normal in infants and children
      • Normal young or slender adults with a horizontally positioned heart can also
        demonstrate a rightward QRS axis on the ECG.
  • Left Axis Deviation (LAD)
    • <-30 degrees
      • moderate LAD: -30 to -45 degrees
      • marked LAD: -45 to -90 degrees
    • Differential diagnosis
      • Left ventricular hypertrophy (LVH)
      • Left Anterior Fascicular Block (LAFB) — diagnosis of exclusion
      • LBBB
      • inferior MI
      • ventricular ectopy
      • paced beats
      • Ventricular pre-excitation (WPW)
      • Primum ASD — rSR’ pattern
  • Extreme Axis Deviation
    • 180 to -90 degrees
      • rare
    • Differential diagnosis
      • Right Ventricular Hypertrophy (RVH)
      • Apical MI
      • VT
      • Hyperkalemia

Rotation

  • can be thought of as the axis of the heart in the transverse axis (the precordial leads)
  • Normal
    • isoelectric QRS in V3 and V4, indicating the transition point between the right and left ventricular electric forces
  • Clockwise rotation
    • isoelectric QRS in V5, V6
  • Anti-clockwise rotation
    • isoelectric QRS in V1, V2

Rule of thumb: the heart rotates towards hypertropy and away from infarction

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